The invention relates to a method for forming channels of high fluid conductivity in formation parts around a borehole penetrating such formation. In particular, the present invention relates to a method for creating channels of high fluid conductivity in relatively hard, acid-soluble formations.
The majority of the known techniques designed for increasing the productivity of formations surrounding a borehole apply a combination of formation fracturing and acid treatment. A propping agent may optionally be added for further increasing the productivity.
A large variety of these techniques are known. In the British Pat. specification No. 672,789, a method is described wherein the formation is fractured by injecting a viscous fracturing medium which, optionally, has a propping agent added thereto. Subsequently, an acid solution may be injected into the fracture to reduce the viscosity of the fracturing medium and to react with the formation.
U.S. Pat. No. 2,689,009 mentions the injection, at fracturing pressure, of a viscous emulsion of an acid and an oily vehicle, which emulsion carries a propping agent followed by the injection of a plain acid for breaking the emulsion and reacting with the formation.
U.S. Pat. No. 3,044,549 describes the injection of a mixture of oil, acid and propping agent at fracturing pressure into an oil-containing formation.
U.S. Pat. Nos. 3,285,340 and 3,481,401 both mention fracturing by means of a viscous medium, followed by the passage of a propping agent into the fracture. Subsequently, the fracture is closed and acid is injected through the fracture containing the propping agent. The agent is either deformable and acid-resistent, or permeable and filled with a surfactant. The agent locally protects the fracture wall against attack by the acid, as a result of which pillars are created in the fracture walls wherein the walls are contacted by the propping agent.
U.S. Pat. No. 3,642,068 describes the creation of a fracture by means of a viscous medium followed by the passage of propping agents into the fracture. The agent is shifted to a remote location in the fracture by means of an acid that etches those parts of the fracture walls which are close to the borehole. Subsequently the fracture is closed.
U.S. Pat. No. 3,842,068 describes the formation of a fracture and the introduction of propping agent into the fracture, followed by the complete closure of the fracture on the propping agent and the injection of acid under conditions at which the fracture remains closed.
Finally, U.S. Pat. No. 3,768,564 mentions the formation of a fracture without using a propping agent, acidizing the open fracture, and continuing said acidization after the fracture has been closed.
Although good results may be obtained by the majority of the above-referred techniques, it is often desirable in oil recovery processes to increase the productivity of a hard, acid soluble formation to a greater extent than is obtainable by the above described techniques.